Hepatitis C virus (HCV) is an important human pathogen that is strongly associated with transfusion related non-A, non-B (NANB) hepatitis. Amplification of HCV RNA sequences by reverse transcription and polymerase chain reaction (cDNA PCR) is the only practical method currently available to demonstrate viremia in patients with HCV infection. However, genetic heterogeneity among different HCV strains results in false negative results in cDNA PCR assays because of primer and template mismatch. HCV accounts for up to 25% of community-acquired hepatitis and over 90% of transfusion- associated hepatitis in the United States. The recent cloning and sequencing of the genome of HCV led to the development of specific serologic tests for antibody to HCV (anti-HCV) that utilized recombinant- derived viral antigens. These tests have been useful for diagnostic and screening purposes but they shed little light on immunity to HCV, in part because currently available antigens are expressed from parts of the HCV genome that encode nonstructural or internal structural proteins. Furthermore, the finding of genetic heterogeneity of the HCV genome, especially in the gene encoding the envelope proteins, suggests that there may be heterogeneity of viral envelope proteins similar to that seen in human immunodeficiency viruses. Such a finding would bode ill for attempts at vaccine development. The objectives of this project are to: (1) identify primer sets that have a low false negative rate in a cDNA PCR assay; (2) determine the nucleotide sequence of diverse HCV genomes; and (3) analyze whether infection of primates with HCV induces protective immunity.